The U.S. Congress recognizes Earth Day, in 1975.

Designating March 21, 1975 as "Earth Day"

Five years after the rest of the nation first observed Earth Day, the U.S. House of Representatives and Senate took up a joint resolution to publicly declare Earth Day as a time to draw national attention to environmental issues. The link below brings you to 4 pages of debate on the issue from the Congressional Record (at the lower right of the first page). These open with the text of the resolution itself (joint resolution 258), and conclude with the roll call vote. The measure passed easily and became law on March 20, 1975.